1. Field of the Invention
The present invention relates to pitch-based activated carbon fibers. More particularly, it pertains to optically isotropic pitch-based activated carbon fibers in which the relative proportion of the number of ultramicropores, pore diameters and pore densities (the number of pores per unit volume of the fiber) are regulated and which selectively exhibit a high adsorption efficiency according to various purposes of use and are particularly suited for adsorbents for low-molecular organic compounds and inorganic compounds, adsorbents for trace amounts of radioactive substances, catalyst carriers, electrodes for secondary batteries and the like.
2. Description of the Related Arts
Particulate activated charcoals and activated carbon fibers have heretofore been known as materials exhibiting the capability of adsorbing and desorbing a variety of substances and ions. In particular, being in the form of fibers, activated carbon fibers have come to be widely used, with or without additional treatment such as shaping, as materials for adsorbing applications such as absorbent, water purifiers, deodorant or deodorizing filters, catalyst carriers and applications making use of the intercalation potential of ions to carbon such as batteries, capacitors or condensers.
In order that the particulate activated charcoals or activated carbon fibers may fully exhibit their adsorption-desorption functions, size and density and/or distribution of the pores as well as structure of the pores are generally considered to be significant factors.
However, adjustment of the diameter, the density and the distribution of the pores is extremely difficult because they are varied depending on raw pitch materials and production conditions. In fact, Japanese Patent Application Laid-Open No. 295218/1986 discloses a technique for controlling the distribution of the pores in an optically isotropic pitch-based activated carbon fiber according to the purpose of applications.
However, nothing is known about conventional particulate activated charcoals or activated carbon fibers in which the distribution of the pores in the inner part of the fiber is controlled, for example, how to achieve a uniform density of the pores.
Taking into consideration the size of low molecular organic compounds, inorganic compounds, metal atoms, ions or the like, activated charcoals or activated carbon fibers each having ultramicropores of 0.5 nm or smaller in pore diameter are expected to find a variety of applications. However, it has been impossible with any of the conventional technique to control the relative proportion of the number of ultramicropores therein. As the result of investigation on various activated charcoals, there has not yet been found an activated charcoal having a proportion of the number of ultramicropores with a pore diameter of 0.5 nm or smaller to the total number of being from 70% to 92% opores with a pore diameter of 4 nm or smaller which proportion exceeds 70%.
When an activated carbon fiber has pores distributed uniformly not only in the surface layer part but also in the inner part of the fiber, the number of the pores in the unit volume of the fiber is increased and accordingly, the efficiency of the adsorption by the fiber is enhanced. The fiber having such a structure is expected to find a still wider range of applications.
However, hitherto none of the conventional particulate activated charcoals and activated carbon fibers has sufficiently met the above-mentioned requirement regardless of the origin such as pitch-based materials or organic materials, including rayon-based, polyacrylonitrile-based, phenol resin-based and the other materials.
The pores can be classified in macropores having a diameter of 50 nm or larger, mesopores having a diameter in the range of 5 to 50 nm, micropores having a diameter in the range of 0.5 to 5 nm and ultramicropores having a diameter of 0.5 nm or smaller.
The pores structures of the conventional particulate activated charcoals and activated carbon fibers are roughly classified in a structure in which macropores are in the surface layer part of the fiber, mesopores are in the inner part thereof and micropores along with ultramicropores are in the further inner part thereof, and a structure in which mesopores are in the surface layer part of the fiber and micropores along with ultramicropores are in the inner part thereof.
It is generally believed that micropores are the most effective for the adsorption. In the conventional materials, the micropores are developed straight forward and are distributed mainly in the part close to the surface of the material and the diameter thereof reduces monotonously with the distance from the surface.. To attain higher adsorption efficiency in this kind of structure, the number of pores in the surface layer must be increased resulting in the problem that the mechanical strength of the material is inevitably deteriorated. In addition, nothing has heretofore been known regarding an activated carbon fiber having ultramicropores of 0.5 nm or smaller in pore diameter as principal pores.
As a result of extensive investigation made by the present inventor in order to solve the above-mentioned problem, it has been found that optically isotropic pitch-based activated carbon fibers are obtained in which the relative proportion of the number of ultramicropores, pore diameters and pore densities are properly regulated; a large number of micropores and ultramicropores that communicate with at least a part of the surrounding pores are distributed with an almost uniform density on the surface layer part and also in the inner part of the fibers; and substantially the micropores having a pore diameter larger than 4 nm are not recognized, by regulating the preparation conditions for optically isotropic pitch, spinning and/or infusibilizing conditions for the pitch fibers, carbonizing conditions and further, activating treatment conditions for the infusibilized pitch fibers and/or the carbonized pitch fibers. The present invention has been accomplished on the basis of the above-mentioned finding.
Specifically, the present invention provides optically isotropic pitch-based activated carbon fibers having a proportion of the number of the ultramicropores with a pore diameter of 0.5 nm or smaller to the total number of pores with a pore diameter of 4 nm or smaller being at least 70%.